A fuel-cell electric vehicle includes a fuel cell that generates electricity from air and a fuel mainly made of hydrogen, an air supplying device for supplying air to the fuel cell, a fuel supplying device for supplying a fuel such as hydrogen to the fuel cell, and a motor for converting electricity into driving power. Such a fuel-cell electric vehicle normally employs a hybrid configuration where both a fuel cell and a secondary battery are provided. For fuel cell systems of electric vehicles, polymer electrolyte fuel cells are mainly used.
As described above, a fuel cell uses a fuel gas such as hydrogen. Since hydrogen has a small molecular weight, it is difficult to completely seal a fuel cell system to prevent leakage of hydrogen. To cope with this problem, a method of detecting leakage of hydrogen gas at an early stage with a hydrogen sensor in a vehicle has been proposed (see patent document 1). Patent document 1 also discloses a method of preventing malfunction of a hydrogen sensor by improving its detection accuracy.    [Patent document 1] Japanese Patent Application Publication No. 2002-352824